It is known to provide vehicles with a transfer case (also referred to as a reduction gearbox) to provide additional drive ranges. The transfer case is typically coupled to an output shaft from a transmission and, in a four-wheel drive vehicle, provides drive to the front and rear axles via front and rear propeller shafts. The torque delivered to the front and rear axles can be the same (i.e. 50:50 distribution) or different. The transfer case can also include a reduction gear set, for example a helical reduction or an epicyclic reduction gear set, to provide high and low drive ranges. The high range can provide a 1:1 ratio (i.e. direct drive) and the low range can provide a reduction gearing, for example 2.69:1.
The transfer case has particular application in off-road vehicles where the low range can increase the torque delivered at the wheels and provide improved low-speed control. High range should be used for all normal road driving and also for off-road driving across dry, level terrain. Low range should only be required where low speed manoeuvring is necessary, such as reversing a trailer, negotiating steep slippery surfaces or boulder strewn terrain. Low range should also be used for extreme off-road conditions where progress in high range cannot be maintained. Low range should not normally be used for normal road driving.
The transfer case can also include a synchroniser to enable dynamic range changes while the vehicle is travelling (referred to as shift-on-the-fly or shift-on-the-move control). By way of example, the transfer box can facilitate range changes when the vehicle is moving, within set limitations as follows:                High to Low—at speeds not exceeding 10 mph (16 km/h)        Low to High—at speeds not exceeding 30 mph (48 km/h).        
If no synchroniser is fitted, static range changes are typically implemented with the vehicle stationary. The transfer case range changes from low to high are referred to herein as upshifts, and the transfer case range changes from high to low are referred to herein as downshifts. The transfer case can also be moved into a neutral position for towing the vehicle.